A METHOD OF MAKING THIN FILM COMPOSITE MEMBRANES

Abstract: A method of preparing a semipermeable composite membrane is disclosed. The composite membrane includes a discriminating layer formed in situ by interfacial polymerization. This type of polymerization most often utilizes a water immiscible solvent for one of the reactants. The method described provides for the removal of the water immiscible solvent by physical means (requiring no phase change). Physicial methods described include a water knife, air knife, roller or roller blade. Composite membranes having good flux, good salt rejection, and low levels of residual organic solvent are obtained.

Description: A METHOD OF MAKING THIN FILM COMPOSITE MEMBRANES This invention relates to permselective barriers in the form of thin films or composites thereof One aspect of this invention relates to aromatic polyamide discriminating layers prepared on porous supports suitable for reverse osmosis desalination of aqueous solutions containing dissolved solutes This invention also relates to a process for the preparation of interfacially polymerized aromatic polyamide discriminating layers on porous supports and to the use of such membranes for the separation of fluid mixtures into components thereof 0 Reverse osmosis membranes, and particularly interfacially polymerized polyamide thin film composite membranes, are known U S Patent 4,277,344 discloses a polyamide thin film composite membrane prepared from polyamine monomer and polyfunctional acyl halide monomer Typically the membranes there provided are useful for the separation of dissolved substances (e g , salts) from solvents (e g , water) by the process of reverse osmosis 5 Interfacially polymerized thin film composite membranes are formed insitu on a supporting surface, e.g , through chain extension and/or crosslinking of monomers and/or prepolymers By forming the thin film polymeric discriminating layer insitu, the thin film need not be soluble in any solvent Interfacially synthesized discriminating layers can thus be cross nked polymers For casting, polymeric discriminating films must necessarily be soluble in o some solvent and consequently must exhibit a low crosslink density Thin film composite membranes now receiving substantial commercial endorsement are obtained by condensing an essentially monomeπc, polyfunctional acyl halide having a low aqueous solubility with a polyamine having a high water solubility at an interface created between the aqueous solution of amme reactant and immiscible solution of polyfunctional acyl halide reactant Interfacial 5 polymerization is also disclosed in U S Patent 4,277,344 as occurring between an aqueous polyamine solution and a vapor phase acyl polyacyl halide reactant A generally more conventional interfacial polymerization technique involves contacting the polyamine solution with a substantially nonpolar organic liquid solution of the polyacyl halide Typical nonpolar solvents disclosed in the literature include C^C aliphatic 0 (including halogenated aliphatic) solvents such as the alkane (including halogenated alkane) solvents and the cyclo aliphatic solvents (typically having five or six carbon atoms and typically being fully saturated or having a very low level of unsaturation) "Liquid" solvents in this context, are solvents which are liquid under normal ambient conditions of temperature and pressure, (e g , 20-25°C and 1 atmosphere) Conveniently used solvents include C -C 5 halogenated ahphatics, C ^ -C alkanes, and C and C ^ cycloa phatics Suitable solvents for the polyacyl halide do not tend to initiate chemical attack or physical attack on commonly used substrate materials The porous substra'e Teαuently is prepared from polysu one Consequently, the chemical compos * ιoπ ot the suostrate may unit the use of some C -C halogenated aliphatics as solvents for the polyacyl halide. Alkane and cyclo alkane solvents do not exhibit strong attack on substrates such as polysulfone. The hydrocarbon solvents including C.-C ]2 aliphatic as disclosed are combustible. 1 ,2,2-trichloro-1, 1 ,2-trifluoroethane is a useful solvent because of its limited toxicity and safety in terms of flammability. Recently, the environmental concerns related to "ozone depletion" have focused on solvents having environmental characteristics more favorable than low molecular weight alkanes, chlorinated alkanes, and volatile fluorochlorocarbon solvents such as are marketed under the E. I. Dupont de Nemours trademark FREON. 1 ,2,2-trichloro-1 ,1 ,2-trifluoroethane is marketed as Freon TF. Although the interfacial polymerization is possible with solvents for the polyacyl halide having higher boiling points such as C 3 -C 12 aliphatic solvents, it is believed that commercial implementation of the process of preparing thin film composite membranes, particularly the preparation of thin film composite polyamide membranes has focused on lower boiling solvents such as Freon TF and hexane in order that the solvent might readily be removed from the membrane surface by evaporation. Such solvent removal methods are disclosed in U.S. Patent 4,277,344; U.S. Patent ,783,344; U.S. Patent 4,749,488; U.S. Patent 4,859,384; U.S. Patent 5,015,382; and U.S. Patent 5,015,380. It is noted in U.S. Patent 4,761 , 234 that n-hexane and trifluoro-trichloroethane are preferred solvents "forthe reaction rate and volatility of the solvent." The preference for trifluoro-trichloroethane because of its low flammability is expressed. Heretofore the art has taught that the use of higher boiling solvents such as C 3 to C 12 alkanes requires the use of extraordinary means, including elevated temperatures, to remove the higher boiling organic solvent by means of a heat-induced evaporation (phase change) in an oven at temperatures from 60 to 120°C. It is disclosed in U.S. Patent 4,948,507 that a monomeric amine salt such as triethylamine camphor sulfonic acid is a necessary ingredient of the aqueous polyamine reactant solution to avoid a loss of flux when the composite membrane is heated at an elevated temperature for the purpose of removing a higher boiling solvent such as lsopar IM G. Isopar ™ is a trademark of Exxon Corporation for high purity isoparaphenic solvents. Higher boiling solvents are more recently suggested as a solution to the environmental risk attributed to chlorofluorocarbon solvents and the safety risk posed by lower boiling al kanes such as n-hexane or cyclohexane. It is disclosed in European Patent 0 503 596 A2 that, in the manufacture of interfacially polymerized thin film composite membranes, a step of evaporating the solvent is required afterthe polyfunctional acyl halide reactant is contacted with the reactive monomer, frequently a polyfunctional amine. The use of higher boiling sol vents for the polyfunctional acyl halide reactant is reported to result in membranes exhibiting a low water flux. It is also reported that the application of heat to assist in the evaporation of the solvent resulted in membranes having extremely low water flux. A solution proposed in EP 0 503 596 A2 is to assist the evaporation of the high boiling solvent Dy blowing humid gas, such as air, over the surface of the membrane The effect of accelerating the evaporation by moving gas over the membrane surface is believed to minimize the adverse effect which the exposure of the membrane to extended elevated temperature has on the flux upon the membrane The gas velocity disclosed is between 2 and 20 m/sec at a temperature of 10to 80°C, provided that the temperature is lower than the flash point of the solvent It is further disclosed in EP 0 503 596 A2 that removal of the solvent by evaporation is necessary to cause the concentration of the crossl inking agent, e g , a polyfunctional acid chloride such as tπmesoyl-chloπde, in order to promote the crosslinking reaction The crosslinking reaction is claimed to be complete when the solvent is completely o evaporated It is further disclosed that even at elevated temperatures, a gas enhanced evaporation of the solvent requires more than a minute of time Reduction of the time necessary for removal of the solvent would result in a more desirable manufacturing process For a solvent having a low volatility, it is stated the reaction time will be too long resulting in an undesirablythick crosslinked layer resulting in a low water flux 5 There is a need for a method of making a reverse osmosis composite membrane having a thin film, interfacially reacted, discriminating layer having a reaction time less than 1 minute There is also a need for a method of removing the solvent which does not require elevated temperatures or additives to the amine reactant solution Applicants have determined that evaporation of the solvent as disclosed in EP 0 0 503 596 and practiced in the art is not necessary The organic solvent may be physically removed from the membrane without the necessity of a phase change When properly directed upon the membrane surface, water or air may be used to quickly and adequately remove at least 70 percent of the organic solvent and the acyl halide reactant without the loss of flux or salt rejection and without a phase change Other physical methods including a roller, or combinations of physical methods, are useful to remove the organic solvent These methods of removal of the water immiscible solvent from the membrane have the particular benefit that the solvent does not change phase, e g , from liquid phase to gas phase on removal from the membrane The physical removal of the water immiscible solvent is contrary to the previously understood belief that the solvent must be evaporated in order to complete the crosslinking step of the membrane formation Preferably, physical methods for removal (in the absence of a phase change) result in displacement of greaterthan 90 percent of the organic solvent and unreacted monomer, more preferably greater than 98 percent, and most preferably greater than 99 percent Additional organic solvent removal may be accomplished by evaporative phase change Removal of solvent by physical means, or by a combination of physical means and evaporation, yields membranes having good flux, and good rejection Furthermore, the method may be used with solvents which are cost effective, environmentally responsible, and have a high flash point The water used for physical removal of the organic solvent can be applied to the membrane at any temperature at which the water is liquid Because the water is typically in contact with the membrane only a short time, little adverse consequences to the polymeric support from an elevated temperature occurs A further feature of the invention is that by use of low temperature water for solvent removal a relatively volatile solvent such as a solvent having a flash point of 30°C or less may be employed with greater safety Low temperature water, i e , less than 10°C may be combined with a comparatively volatile organic solvent having a low flash point in a safe manner By removing the solvent in the liquid phase, the occurrence of an explosive vapor mixture of solvent and air may be avoided, even though the temperature is above the flash point This may permit the use of solvents having flash points less than 0°C including hexane and cyclohexane The Applicants' discovery eliminates the need for the addition of drying aids such as monomeπc amine salts The Applicants' discovery also eliminates the need for extraordinary safety measures required when heating the solvent laden membrane The present invention concerns semipermeable membranes comprising a microporous substrate and an ultra-thin membrane coated on the microporous substrate wherein the ultra-thin layer comprises a crosslinked polyamide Polymers other than polyamides such as polyurea, are also contemplated In a preferred embodiment of the invention, a microporous support is coated with a polyfunctional reactant such as a polyamine The polyamine may be primary or secondary in nature and may be selected from aliphatic or aromatic polyamines The aliphatic polyamine may be linear or cyclic Useful aliphatic polyamines include ethylenediamine, polyethylenediamine, n-cyclohexanediamine, cyclohexanetπamine, and piperazine Useful aromatic polyamines include phenylenediamine such as m-phenylenediamine, p-phenylenediamine, and tπaminobenzene A preferred form of polyamine has an aromatic nucleus of one or more aromatic rings Aromatic polyamines having a plurality of aromatic rings are preferably independent rings rather than fused rings To aid in solubility, there is advantageously a "bridge" between aromatic rings Preferably, the bridge has a "polar" or "hydrophilic" character The aromatic nucleus of the polyamine may be carbocyclic, or heterocyclic Substituted polyamines are also useful if the substituents do not interfere with the formation of crosslinks in the thin film discriminating layer Substituents such as -H, OCH 3 , -CH 3 , C 2 H 5 , -C_H,, -S0 3 H, -COOH, -NHCH 3 , -COCH , -F, -CI, and -Br are useful Substitution immediately adjacent to a carbonyl or amine group of an amide bond is not suggested because of steπc interference with the desired reaction The degree of functionality of the amine reactant is at least two The reactant so tion may be comprised of a mixture of reactants A mixture of polyfunctional reactants should have an average functionality greater than two Water is the most ad antageous solvent for the polyamine reactant However, solvents other than water which dissolve from 0 5 to 40 percent polyamine are within the scope of the invention Such solvents include polyethylene glycols The organic soluble reactant is advantageously one or more acyl ha des having an average functionality greater than or equal to two Suitable polyfunctional compounds reactive with the polyfunctional amine reactants include diisocyanates, for example aliphatic diisocyanate such as diisocyanotohexane and preferably aromatic diisocyanates such as toluene diisocyanate Also preferred are acyl ha des, (including their functional equivalents such as acid anhydrides) such as isophthalyl chloride, terephthaloyl chloride, and tπmesoyl chloride Bridge linkage between aromatic nuclei may comprise a single bond, or a divalent group capable of bonding two aromatic rings, for example -NHCO-, -SO.,-, -S-, -COO-, - CH 3 CCH 3 -, and -O- Suitable solvents for the acyl hahdes are nonpolar liquids such as substituted or unsubstituted aliphatic, a cyclic, or aromatic hydrocarbons in which the acyl ha des have a solubility of at least 0 01 percent by weight Typical solvents for polyacyl halide are C 5 to C 8 alkanes Halogenated alkanes ( C^o C, ) have also been used Alkanes up to C ]2 are disclosed in U S Patent 4,277,344 as suitable solvents for polyacyl hahdes In commercial practice the volatility of the organic solvent has been a significant feature as stated in U S Patent 4,761 ,234 The safety advantages of 1 ,1,2-trιchloro-1 ,2,2-trιfluoroethane as solvent for acyl hahdes is also stated in U S Patent 4,761 , 234 Useful solvents in the present method of manufacture include the solvents heretofore disclosed A useful feature of the present method is its utility with solvents having low vapor pressures For flammable solvents, low vapor pressures frequently correspond to safer high flash points (the temperature at which the vapor forms an ignitable mixture in air near the liquid) Thus, the present method is useful for the physical removal of organic solvents having vapor pressures at 38°C of (30 kPa and lower even to 5 kPa, and as low as) 0 5 kPa or lower It has now been found that solvents having a low volatility may be used and that the resulting flux of the thin film composite membrane is influenced by the residual solvent Advantageously the associated solvent, adsorbed on the surface and/or contained in the barrier layer is preferably less than 1000 mg/m 2 , more preferably less than 500 mg/m * , even more preferably less than 100 mg/m 2 , and most preferably less than 50 mg/m 2 Various physical methods are effective for removal of low volatility solvents Scraping, water sprays or water knifes, gas sprays, and nip rolling are found effective A means of removing organic solvent such as a "nip" may include a turn roller on a continuous membrane sheet wherein the memorane layer having the discriminating layer thereon iS m tension against a turn roller The expression "water knife" is not limited to a water soray τ rom a si ngle nozzle or water source but includes a plurality of water sources and may include a spray pattern which is linear, oval, circular, or any combination thereof. Table 1 shows a variety of physical methods for removal of low volatility solvents and the comparative effectiveness of each method. Examples: Comparative Example 1 Composite reverse osmosis membranes were prepared starting from generally well-known methods. Microporous supports may be prepared from a variety of polymeric materials in addition to polysulfone, including polyethersulfone, chlorinated polyvinyl chloride, polystyrene, and cellulose esters. The pore sizes of suitable supports is from 0.001 to 1 μπn. Preferably the pore size, as determined by electron microscope on the surface in contact with the thin film discriminating layer, is from 0.01 to 0.1 μm. Supports prepared from polysulfone are widely used commercially and were used herein Preparation of suitable supports is described in U.S. Patents 3,926,798 and 4,039,440, and in Office of Saline Waters R&D Progress Report No. 359 of October 1968. The disclosure of these references is i ncorporated herei n by reference i n thei r enti rety. The membrane support was then contacted with an aqueous solution of 2.5 percent by weight of m-phenylenediamine. After nip rolling, the exposed surface of the microporous support was then contacted with 0.12 percent by weight solution of trimesoyl chloride in a solvent immiscible with water. A thin film discriminating layer formed at the interface of the solution of the polyfunctional compound and the support. The organic solvent for the trimesoyl chloride was n-dodecane. Example 1 serves as a control. No physical removal of the solvent was attempted. All examples were performed in triplicate unless otherwise noted. Results of reverse osmosis testing at 225 psig with 2000 ppm NaCI and residual hydrocarbon are reported in Table 1 for Examples 1 to 8. Residual hydrocarbon was determined by gas chromatography. Example 2 A composite membrane was prepared according to the procedure of Example 1 except that the composite membrane after 1 minute reaction time was immersed for 5 seconds in a water bath. Example 3 A composite membrane was prepared according to Example 1 except that immersion in water was performed after physical solvent removal . Solvent removal was accomplished with a red silicone rubber blade. The blade was advanced at about 2 cm/sec. The applied force was approximately 20.5 g/cm of length of the rubber blade. Example 4 A composite membrane was prepared according to Example 1 except that the solvent was removed with the use of a red silicone rubber roller. The roller was applied with a force of 0.3 kg/cm. The roller contacted the composite membrane for about 4 mm of the roller circumference. The roller was advanced along the membrane at about 2 cm/sec. Example 5 A composite membrane was prepared according to Example 1 except that a gas spray using air was used to physically remove the organic solvent. Such a gas spray is frequently called an air knife. The air knife was advanced past the membrane at a rate of about 2 cm/se . At the membrane surface, the ambient air had a velocity measured near the membrane surface greater than 100 ft/sec. Example 6 A composite membrane was prepared according to Example 1 except that a water knife was used to physically remove the solvent. The water knife used in this case consisted of a fan of water from a nozzle. The water nozzle (Spraying System Co. Fiat Jet, 1/4P 3510, 316SS) had a 35° spread. Water was sprayed on the thin film discriminating layer from a distance of 20 cm. The water knife impacted the membrane perpendicular to the membrane surface. The water was at ambient temperature. The membrane advanced lengthwise under the water knife at 16 cm/sec. The water nozzle was pressurized to 276 kPa (40 psig). The water flowed on the membrane surface at a volume of 0.063 liter/sec (1 gpm) per nozzle or 0.005 liter per seconds per cm of membrane width. Example 7 A composite membrane was prepared according to Example 4. After rolling the thin film composite layer according to the procedure of Example 4 the membrane was washed with the water knife according to Example 6. Comparative Example 8 A thin film composite membrane was prepared according to Example 1 except hexane was substituted for dodecane. The hexane was permitted to evaporate for 2 minutes following the polyamide forming reaction. The membrane appeared dry after 45 seconds. Table 1 Dodecane St. Dev. Flux St. Dev. Salt St. Dev. Physical Example Remaining (mg/m2) (mg/m2) (GFD) (GFD) Passage (%) Method (%) 1 1 21807 2379 9.96 1.66 1.92 1.07 None 1 2 12840 1893 NT - - - water bath 3 1296 350 12.42 2.53 10.89 1.75 red silicone rubber blade I 4 289 - 7.32 3.69 1.22 0.42 red silicone ∞ rubber roller 5 292 75 14.86 0.56 1.31 0.38 air knife 6 47 22 14.00 1.90 2.63 - water knife 7 63 48 1 1.95 1.09 1.73 1.18 roller and water knife Comp. 8 1 * 2 2 2 9.43 0.77 1.13 0.1 5 evaporation of hexane 1 Not an example ot the invention. 2 Hexane was the solvent used here Examples 10-13 A composite membrane was prepared following the steps of Example 6. Using a water knife having a constant flow rate, water pressure and water volume, the efficiency of solvent removal was determined with respect to the distance of the membrane from the water 5 knife. In all cases the membrane was perpendicular to the water knife. Results are in Table 2. Individualized conditions may change the results of efforts to remove solvent from a composite membrane. For example, the particular solvent chosen, the water knife used, the water flow rate, the water knife angle with respect to the membrane, and the distance of the membrane from the water knife may impact the results obtained. Absolute flux and salt 10 passage values depend on many other criteria as well (e.g., the concentration of reactants). Thus, for comparative purposes, all reactions were held at equivalent conditions. Table 2 St. 15 Ex. Distance Dodecane Remaining St. Dev. Flux St. Salt (cm) (mg/m2) age Dev. (mg/m2) (GFD) Dev. Pass (GFD) (%) (%) 10 20 47 22 14.00 1.90 2.63 - 11 40 353 148 13.69 1.96 2.98 0.17 12 60 2569 417 11.16 1.03 2.93 0.75 20 13 80 2968 595 9.07 0.77 3.20 0.46 Examples 14-17 -m. Composite membranes were prepared according to Example 6 except that n- dodecane was replaced by the solvent indicated in Table 3. The residual solvent and flux appears in Table 3. 30 5 Table 3 Solvent Salt andard Flux Standard Standard Example Solvent Remaining St assage (mg/m2) Deviation (GFD) P Deviation Deviation (%) 6 dodecane 47 22 14.00 1.90 2.63 — 14 decalin 29 16 10.17 4.70 3.03 1.16 15 n-nonane 27 8 22.44 0.53 1 .82 0.75 16 n-tetradecane 901 185 13.49 1.89 1.54 1.09 17 Isopar™ L 15 18 16.67 1.80 1.49 0.44 Isopar™ is a trademark of the Exxon Company for an isomeric mixture of isoparafins. ? Examples 18-29 Composite membranes were prepared according to Example 6 except the water knife angle of incidence with the membrane surface, the water pressure, and the water nozzle were varied as shown in Table 4. Table 4 Residual Example Water essure Standard Nozzle Angle Pr Average Deviation No. No. (psi) (mg HC/m 2 ) (mg HC/m 2 ) 18 3510 15 10 215 167 19 3510 90 10 301 186 20 3510 15 40 27 12 21 3510 90 40 47 22 22 5010 15 10 457 1 23 5010 90 10 4930 1207 24 5010 15 40 19 2 25 5010 90 40 99 50 26 3504 15 10 599 427 27 3504 90 10 2732 370 28 3504 15 40 33 30 29 3504 90 40 309 105 Nozzle 3510 has a 35° spray angle at 276 kPa (40 psi) and 0.063 liter/second (1 gallon per minute). Nozzle 3504 has a 35° spray angle at 40 psi and 0.4 gallon per minute. Nozzle 5010 has a 50° spray angle at 40 psi and 1 gallon per minute.

Claims: WHAT IS CLAIMED IS 1 A process for preparing a composite membrane comprising a) coating a porous support with a first solution comprising a polyfunctional reactant and water, b) contacting the first solution which is on the porous support with a second solution, which is immiscible with the first solution, the second solution comprising a second solvent and a polyfunctional compound reactive with the polyfunctional reactant, such that an interfacial reaction occurs between the polyfunctional reactant and the polyfunctional compound forming a film on the porous support, and c) removing, without a phase change, at least 70 percent of any excess polyfunctional compound and second solvent from the porous support and film by physical means 2 The method of Claim 1 wherein the physical means of removal of step (c) is selected from the group consisting of water spray, gas spray and nip roller 3 The method of Claim 2 wherein the physical means is a water spray applied such that an angle of incidence of between 5 and 25° is formed by the water spray and the surface of the composite membrane 4 The method of Claim 3 wherein the water spray is at a temperature of less than 30°C 5 The method of Claim 3 wherein the water temperature is less than 10°C and the solvent has a flash point less than 0°C 6 The method of Claim 2 wherein the gas spray contains nitrogen 7 The method of Claim 1 wherein, without a phase change, at least 90 percent of any excess polyfunctional compound and second solvent is removed from the porous support and film thereon by physical means 8 The method of Claim 7 wherein, without a phase change, at least 98 percent of any excess polyfunctional compound and second solvent is removed from the porous support and f'lm thereon by physical means 9 A composite membrane comprising a porous support and an interfacially polymerized discriminating layer prepared according to the method of Claims 1 to 8

Inventors: ANSCHUTZ JAMES E, BATZEL DANIEL A, HALLFRISCH VIRGINIA A, JOHNSON DAVID R, STUTTS KENNETH J, ANSCHUTZ, JAMES, E., BATZEL, DANIEL, A., HALLFRISCH, VIRGINIA, A., JOHNSON, DAVID, R., STUTTS, KENNETH, J.
Applicants: DOW CHEMICAL CO, THE DOW CHEMICAL COMPANY
Application year: 1994
Main classification: B32B3
IPC codes: B01D69, B32B27, C08J9